Method for the assessment of neuromuscular integrity and burrowing choice in vermiform animals

BACKGROUND

The study of locomotion in vermiform animals has largely been restricted to animals crawling on agar surfaces. While this has been fruitful in the study of neuronal basis of disease and behavior, the reduced physical challenge posed by these environments has prevented these organisms from being equally successful in the study of neuromuscular diseases. Our burrowing assay allowed us to study the effects of muscular exertion on locomotion and muscle degeneration during disease (Beron et al., 2015), as well as the natural burrowing preference of diverse Caenorhabditis elegans strains (Vidal-Gadea et al., 2015).

NEW METHOD

We describe a simple, rapid, and affordable set of assays to study the burrowing behavior of nematodes and other vermiform organisms which permits the titration of muscular exertion in test animals.

RESULTS

We show that our burrowing assay design is versatile and can be adapted for use in widely different experimental paradigms.

COMPARISON WITH EXISTING METHOD(S)

Previous assays for the study of neuromuscular integrity in nematodes relied on movement through facile and homogeneous environments. The ability of modulating substrate density allows our burrowing assay to be used to separate animal populations where muscular fitness or health are not visible differentiable by standard techniques.

CONCLUSION

The simplicity, versatility, and potential for greatly facilitating the study of previously challenging neuromuscular disorders makes this assay a valuable addition that overcomes many of the limitations inherent to traditional behavioral tests of vermiform locomotion.

The burrowing behavior of the nematode Caenorhabditis elegans: a new assay for the study of neuromuscular disorders

The nematode Caenorhabditis elegans has been a powerful model system for the study of key muscle genes relevant to human neuromuscular function and disorders. The behavioral robustness of C. elegans, however, has hindered its use in the study of certain neuromuscular disorders because many worm models of human disease show only subtle phenotypes while crawling. By contrast, in their natural habitat, C. elegans likely spends much of the time burrowing through the soil matrix. We developed a burrowing assay to challenge motor output by placing worms in agar-filled pipettes of increasing densities. We find that burrowing involves distinct kinematics and turning strategies from crawling that vary with the properties of the substrate. We show that mutants mimicking Duchenne muscular dystrophy by lacking a functional ortholog of the dystrophin protein, DYS-1, crawl normally but are severely impaired in burrowing. Muscular degeneration in the dys-1 mutant is hastened and exacerbated by burrowing, while wild type shows no such damage. To test whether neuromuscular integrity might be compensated genetically in the dys-1 mutant, we performed a genetic screen and isolated several suppressor mutants with proficient burrowing in a dys-1 mutant background. Further study of burrowing in C. elegans will enhance the study of diseases affecting neuromuscular integrity, and will provide insights into the natural behavior of this and other nematodes.

Coding characteristics of spiking local interneurons during imposed limb movements in the locust

The performance of adaptive behavior relies on continuous sensory feedback to produce relevant modifications to central motor patterns. The femoral chordotonal organ (FeCO) of the legs of the desert locust monitors the movements of the tibia about the femoro-tibial joint. A ventral midline population of spiking local interneurons in the metathoracic ganglia integrates inputs from the FeCO. We used a Wiener kernel cross-correlation method combined with a Gaussian white noise stimulation of the FeCO to completely characterize and model the output dynamics of the ventral midline population of interneurons. A wide range of responses were observed, and interneurons could be classified into three broad groups that received excitatory and inhibitory or principally inhibitory or excitatory synaptic inputs from the FeCO. Interneurons that received mixed inputs also had the greatest linear responses but primarily responded to extension of the tibia and were mostly sensitive to stimulus velocity. Interneurons that received principally inhibitory inputs were sensitive to extension and to joint position. A small group of interneurons received purely excitatory synaptic inputs and were also sensitive to tibial extension. In addition to capturing the linear and nonlinear dynamics of this population of interneurons, first- and second-order Wiener kernels revealed that the dynamics of the interneurons in the population were graded and formed a spectrum of responses whereby the activity of many cells appeared to be required to adequately describe a particular stimulus characteristic, typical of population coding.

Muscular anatomy of the legs of the forward walking crab, Libinia emarginata (Decapoda, Brachyura, Majoidea)

Decapod crustaceans have been the focus of neuroethological studies for decades. With few exceptions, however, their musculature remains scarcely described. We study the neuroethology of legged locomotion in the portly spider crab, Libinia emarginata (Brachyura, Majoidea), which preferentially walks forward. Majoid crabs are thought to be among the first to have adopted the crab form (carcinification) from lobster-like ancestors, making them interesting subjects for comparative and phylogenetic studies. The radial arrangement of the legs around the thorax, coupled with its unidirectional walking modality makes L. emarginata a good candidate for the presence of anterior and posterior limb specializations. Here we describe the complete muscular anatomy of all the pereopods of L. emarginata and compare our findings with other decapods described in the literature. The number of proximal muscle bundles differs between the anterior and posterior pereopods of L. emarginata. We describe an intersegmental bundle of the flexor muscle similar to the one present in distantly related, forward walking macruran species. The behavioral repertoire, amenability to experimental investigations, and phylogenetic position make spider crabs useful species for the study of the neural control of legged locomotion. To our knowledge, this is the first instance of a complete description and comparison of the musculature in all the locomotor appendages of one species.

Skeletal adaptations for forwards and sideways walking in three species of decapod crustaceans

Crustaceans have been successfully employed to study legged locomotion for decades. Most studies have focused on either forwards-walking macrurans, or sideways-walking brachyurans. Libinia emarginata is a Majoid crab (Brachyura) and as such belongs to the earliest group to have evolved the crab form from homoloid ancestors. Unlike most brachyurans, Libinia walks forwards 80% of the time. We employed standard anatomical techniques and motion analysis to compare the skeleton, stance, and the range of motion of the legs of Libinia to the sideways-walking green shore crab (Carcinus maenas), and to the forwards-walking crayfish (Procambarus clarkii). We found animals tended to have greater ranges of motion for joints articulating in the preferred direction of locomotion. Leg segments proximal to such joints were comparatively longer. Thorax elongation, leg length and placement at rest also reflected walking preference. Comparative studies of walking in Libinia and other brachyurans may shed light on the neuroethology of legged locomotion, and on the anatomical and physiological changes necessary for sideways-walking in crustaceans.